McDonald’s fans who’ve been clamoring to know what’s in Big Mac sauce for years may have been disappointed to find out that it's basically a souped-up Russian Dressing. Yet, Australians are willing to big bucks for it. The fast food chain is auctioning off 200 bottles of “Limited Edition Big Mac Special Sauce” on eBay—starting at Aus$23,100—or about $18,000. […] The proceeds from the auction will benefit Ronald McDonald charities run by the chain that help sick children and their families. So far, over 130 bids have been placed. …